This invention relates to apparatus and method for sustaining the operation of HID lamps and, more particularly, to apparatus and method for preventing HID lamps from extinguishing during relatively short periods of power interruption or drop in voltage so that the lamps immediately operate with full power as soon as the normal line voltage is restored.
High-intensity-discharge lamps are widely used for lighting streets, highways, parking lots, stadiums, high-bay factories and more recently have come into use for department store and office lighting. The term HID lamps includes within its scope high-pressure mercury-vapor lamps, mercury-metal halide lamps and high-pressure sodium lamps which incorporate a filling of mercury and sodium. Upon being turned on, all of these lamps require several minutes to achieve their normal operating brightness, during which period of time the discharge-sustaining constituents are building up in pressure. Upon momentary power interruptions or an appreciable drop in line voltage, the lamps will extinguish. Once extinguished, the lamps cannot again be started with the normal starting and ballasting apparatus until the pressure of the discharge-sustaining constituents has dropped and the lamp can be reignited. For most types of HID lamps, this will take several minutes and it frequently can be as much as 10 to 15 minutes before the lamps are operating normally. The effect of a momentary power interruption is most noticeable in a sports arena or stadium since a nationally televised sports program can be interrupted for perhaps 10 to 15 minutes, but the effect is equally bothersome for persons in a supermarket, office, high-bay factory or even a mine which is lighted with HID lamps.
It is possible to restrike HID lamps immediately after they have been extinguished by the application of an extremely high voltage, such as 10 to 20 KV pulses, across the lamp electrodes, but this is not commercially practical for most types of such lamps since a double-ended construction is required to prevent the applied high voltage pulses from short circuiting across the lead-in conductors. In addition, apparatus for generating such extremely high voltage pulses is quite expensive. For this reason, where standby lighting is required, it normally has been customary to provide some sort of standby incandescent lamp which provides a very reduced level of illumination until the HID lamps can be reignited and are again operating with normal power input.
Standby lighting systems have been provided for fluorescent lamps which can be restruck immediately after they are extinguished since these are a low-pressure discharge device and are not required to cool down before being reignited. For fluorescent fixtures, there are presently commercially available standby battery and inverter packs which can provide standby lighting for an individual fixture during periods of power interruption and these battery and inverter packs are designed to fit into or onto an individual fixture. As noted hereinbefore, however, the problems associated with providing standby lighting power for fluorescent lamps are completely different from those encountered with high-intensity-discharge lamps because of the very difficult reignition problems associated with HID lamps.